Tm2+luminescent materials for solar radiation conversion devices

ABSTRACT

A solar radiation conversion device is described that uses a luminescent Tm2+ inorganic material for converting solar radiation of at least part of the UV and/or visible and/or infrared solar spectrum into infrared solar radiation, preferably the infrared solar radiation having a wavelength of around 1138 nm; and, a photovoltaic device for converting at least part of the infrared solar radiation into electrical power.

FIELD OF THE INVENTION

The invention relates to broadband-absorbing Tm²⁺ luminescent materialsfor solar radiation conversion devices, and, in particular, though notexclusively, solar radiation conversion devices such as photovoltaiccells or luminescent solar concentrators that comprise abroadband-absorbing Tm²⁺ luminescent material, the use of Tm²⁺luminescent materials in solar radiation conversion devices, methods ofsynthesizing Tm²⁺ based luminescent materials and a method for forming aTm²⁺ luminescent (poly)crystalline thin-film layer on a substrate.

BACKGROUND OF THE INVENTION

Solar radiation conversion devices such as luminescent solarconcentrators (LSCs) aim at lowering cost of solar energy generation byconcentrating sunlight using a cheap luminescent plate onto small areastrip photovoltaic cells. In such scheme sun light is absorbed by aluminescent material and re-emitted in all directions. A considerablefraction of the light is trapped in the plate that acts as a light guideby total internal reflection. This way the light is guided to theperimeter where photovoltaic cells convert it into electric power.Similarly, simple luminescent conversion layers(LCL) on top of orintegrated in a photovoltaic device, e.g. a solar cell, may be used toincrease the overall conversion efficiency of the device. In such scheme(part of the) sun light is absorbed by a luminescent material andre-emitted in all directions. A considerable fraction of the light iscoupled into the device either directly or indirectly by total internalreflection. This way the absorption efficiency in the cell is enhanceddue to an enhanced optical path length.

While solar panels are already contributing to the world energyproduction, LSCs and LCLs are still not commercially available. Althoughthe concept is appealing, the production of large sized LSCs and LCLswith sufficient efficiency turns out to be a real challenge. Theproblems that are encountered mainly concern the shortcomings associatedwith the luminescent materials that are used in an LSC or LCL.

Ideally, a luminescent material for an LSC or LCL should meet certainrequirements. The material should have: a broad spectral absorption, ahigh absorption efficiency over the whole absorption spectrum,non-overlapping absorption and emission spectra (i.e. a large Stokes'shift), a high luminescent quantum efficiency (percentage of absorbedphotons that lead to a newly emitted photon), a photon emission thatmatches the spectral response of the PV-cell it is coupled to and itshould be compatible with materials that form the waveguide.

The most common luminescent material that is used for LSCs are organicdyes. These materials are relatively cheap, easily produced and can beintegrated into a waveguide structure in a simple way. The width oftheir absorption spectrum however is limited and the absorption andemission spectra overlap resulting in substantial losses due toself-absorption.

An interesting group of luminescent materials that are currentlyinvestigated for applications in LCSs are the inorganic rare-earthcompounds. These materials can exhibit relatively large shifts betweenabsorption and emission. For example, in the article by De Boer et al,“Progress in phosphors and filters for luminescent solar concentrators”,1202, a phosphor based on Sm²⁺ is described that exhibits emissionaround 700 nm and absorption edge below 600 nm.

Although this material looks promising in view of the re-absorptionproblems of the conventional dyes, the absorbing properties arerelatively poor as it only covers a part of the total solar spectrumthat is available for conversion. Hence there is a need for improvedluminescent materials that both exhibit broadband absorption (i.e.absorption over a large part of the solar spectrum), littleself-absorption and an emission spectrum that may be easily matched withphotovoltaic devices.

SUMMARY OF THE INVENTION

It is an objective of the invention to reduce or eliminate at least oneof the drawbacks known in the prior art. In a first aspect the inventionmay relate to a solar radiation conversion device comprising a Tm²⁺based inorganic luminescent material for converting solar radiation ofat least part of the UV and/or visible and/or infrared solar spectruminto infrared solar radiation, preferably said infrared solar radiationhaving a wavelength of around 1138 nm; and, a photovoltaic device forconverting at least part of said infrared solar radiation intoelectrical power.

The Tm²⁺ based inorganic luminescent material exhibits a large Stoke'sshift so the problem of self-absorption in the solar radiationconversion device does not occur. The Tm²⁺ based inorganic luminescentmaterial absorbs more than twice the amount of power from the sunlightspectrum when compared to absorption of solar radiation of conventionalluminescent materials such as dyes. Contrary to the well-known dye's,Tm²⁺ based luminescent materials are colourless which enhancesapplicability in the build environment. Further, the infrared emissionpeak at 1138 nm of the Tm²⁺ based inorganic luminescent materialsadvantageously coincides with the 1.13 eV bandgap for optimal conversionof the broad solar spectrum (in particular the AMI1.5 solar spectrum) onthe basis of a single-junction cell. This feature allows for improvingthe overall conversion efficiency of a LSC or LCL.

In an embodiment, said luminescent material may comprise a binary,ternary and/or a quaternary inorganic crystalline host material that isdoped with Tm²⁺ ions. In an embodiment said luminescence (emission)originations from Tm²⁺ ions. In another embodiment, said Tm²⁺ ions ispresent in a concentration between 0.1 and 100%. In yet anotherembodiment, in a concentration between 1% and 90%. In a furtherembodiment, between 1% and 50% an in yet a further embodiment between0.2% and 11%.

In an embodiment, said binary inorganic host material may be defined bythe general formula ML wherein M=Na,K,Rb,Cs and L=Cl,Br,I. In anotherembodiment, binary inorganic host material may be defined by the generalformula NL₂ wherein N═Mg,Ca,Sr,Ba and L=Cl,Br,I,F. In yet anotherembodiment, said binary inorganic host material may be defined by thegeneral formula NI₂ wherein N═Mg,Ca,Sr,Ba. Many host materials may beused for the Tm²⁺ ions. It has been found that in particular CaI₂ or NaImay be used as Tm²⁺ inorganic luminescent converter material. Thesematerials exhibit superior absorption characteristics for the solarspectrum.

The host materials may also be an alloy or mixture described by thegeneral formula M1_((1-x))M2_((x))L with M1 and M2 any of the elementsM, or ML1_((1-x))L2_((x)) with L1 and L2 any of the elements L. The hostmaterials may also be an alloy or mixture of the general formulaM1_((1-x))M2_((x))L1_((1-y))L2_((y)). The host materials may also be analloy or mixture of 3 or more elements M or L. The host materials mayalso be an alloy or mixture of the general formula N1_((1-x))N2_((x))L2with N1 and N2 any of the elements N, or NL1_((1-x))L2_((x)) with L1 andL2 any of the elements L. The host materials may also be an alloy ormixture of the general formula N1_((1-x))N2_((x))L1_((1-y))L2_((y)). Thehost materials may also be an alloy or mixture of 3 or more elements Nor L.

In an embodiment, said luminescent Tm²⁺ doped inorganic material maycomprise a (poly)crystalline thin-film layer or crystalline particles,preferably nanoscale particles, wherein said particles are embedded in amatrix material. Tm²⁺ based inorganic luminescent material may besynthesized using different methods, including deposition methods thatare compatible with conventional semiconductor processing methods sothat the Tm²⁺ based inorganic luminescent material may be easilyintegrated in thin-film photovoltaic devices.

In an embodiment said luminescent Tm²⁺ doped inorganic material may bepart of or associated with a waveguide structure for guiding said solarradiation of a predetermined wavelength to said photovoltaic device.

In an embodiment, said waveguide structure may comprise a first (top)surface and a second (bottom) surface, wherein a luminescent Tm²⁺ layeris provided over at least part of said first and/or second surface,preferably said layer comprising a (poly)crystalline thin-film layer ora layer of a matrix material in which crystalline (nanosized) particlesare embedded. Here, the term nano-sized particles may refer to particlesthat have an average size of selected between 1 and 1000 nm, preferablybetween 2 and 800 nm, more preferably between 3 and 500 nm. In a furtherembodiment, the particles may be selected between 25 and 600 nm,preferably between 50 and 200 nm. The nanosized particles provide theeffect that the losses due to scattering in the device are reduced.

In an embodiment, said waveguide structure may comprise a first (top)surface and a second (bottom) surface, wherein said luminescent Tm²⁺material may be embedded e.g. as (nanosized) particles in said waveguidestructure. In an embodiment said matrix material may be a transparentorganic polymer. In an embodiment, said transparent organic polymer maybe a poly(methyl methacrylate) (PMMA) or a polycarbonate. Hence, theTm²⁺ based inorganic material as especially suitable for use inluminescent solar energy concentrator.

In an embodiment, said solar radiation conversion devices comprises awavelength conversion layer wherein said luminescent Tm²⁺ inorganicmaterial is provided over a light receiving face of said photovoltaicdevice. In an embodiment said photovoltaic device may comprise saidluminescent Tm²⁺ inorganic material. In an embodiment, said photovoltaicdevice may comprise a thin-film layer comprising said luminescent Tm²⁺inorganic material.

In an embodiment, said photovoltaic device may comprise an infraredabsorbing active layer, more preferably said infrared absorbing layercomprising at least one of: a type IV, III-V, or II-VI semiconductorcompound, copper indium gallium (di)selenide (CIGS), copper indium(di)selenide (CIS), infra-read absorbing quantum dots, an infraredabsorbing polymer, graphene or (carbon) nanotubes. Hence, the Tm²⁺ basedinorganic material may be used to extend the efficiency of a simpleinfrared photovoltaic device thereby effectively converting the infraredphotovoltaic device into broad-band solar radiation conversion device.

In an embodiment, in said Tm²⁺ luminescent material solar radiationabsorption takes place in the 5d configurations (5d states) of Tm²⁺while the emission (luminescence) is from the Tm²⁺ 4f¹³(²F_(5/2)) to theTm²⁺ 4f¹³(²F_(7/2)) ground state.

In a further aspect, the invention may relate to the use of aluminescent Tm²⁺ inorganic crystalline material in a luminescent solarenergy concentrator or a solar cell.

In another aspect, the invention may relate to a method for synthesisinga luminescent Tm²⁺ doped inorganic crystalline material, wherein themethod may comprise: melting an amount of at least a first inorganicionic compound with a second inorganic ionic compound, said second ioniccompound comprising a Tm²⁺ cation in order to form a Tm²⁺ doped a firstionic compound; and, during said melting maintaining the pressure below5·⁻⁴ mbar, preferably below 1·⁻⁴ mbar.

In yet another aspect, the invention may relate to a method of forming aluminescent Tm²⁺ doped inorganic polycrystalline thin-film on asubstrate, wherein the method may comprise: providing a first sputteringtarget with first sputtering material comprising a first inorganic ioniccompound and a second sputtering target with a second sputteringmaterial is thulium; introducing a gas into the sputtering chamber;heating said substrate to a temperature between 10 and 700° C.,preferably 10 and 600° C.; and, applying an RF electric potential tosaid first sputtering target and a DC electric potential to said secondsputtering target, thereby causing sputtering of said first and secondmaterial from said first and second targets onto said substrate in orderto grow a (poly)crystalline thin-film of said first inorganic ioniccompound that is doped with Tm²⁺ cations.

The invention will be further illustrated with reference to the attacheddrawings, which schematically will show embodiments according to theinvention. It will be understood that the invention is not in any wayrestricted to these specific embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A-1C depict the absorption, excitation and emission spectra ofTm²⁺ doped inorganic materials according to various embodiments of theinvention.

FIG. 2 depicts the absorption spectra of the Red305 dye and theabsorption spectrum of Tm²⁺ doped CaI₂.

FIG. 3 depicts the loss factor due to self-absorption of a Red305 dye asa function of the diameter of a circular LCS.

FIGS. 4A and 4B show SEM pictures of a crystalline Tm²⁺ doped thin-filmlayers according to various embodiments of the invention.

FIG. 5 depicts an XRD measurement of a sputtered Tm²⁺ doped NaCl thinfilm according to an embodiment of the invention.

FIG. 6 depict excitation spectra of an Tm²⁺:NaCl thin film and of aTm²⁺:NaCl powder sample according to an embodiment of the invention.

FIG. 7 depicts the emission spectrum of Tm²⁺:NaCl thin-film and of aTm²⁺:NaCl powder sample.

FIG. 8 depicts a fit of transmission spectrum of a Tm²⁺ doped NaCl filmaround the 4f¹³-4f¹³ Tm²⁺ absorption region.

FIG. 9A-9C depict schematic cross-sections of luminescent solar energyconcentrators comprising a Tm²⁺ doped inorganic material accordingvarious embodiments of the invention.

FIGS. 10A and 10B depict cross-sections of schematic solar radiationconversion devices according to another embodiment of the invention.

DETAILED DESCRIPTION

In this disclosure divalent thulium (Tm²⁺) based inorganic luminescentmaterials are described that have superior and improved properties whencompared to other rare-earth doped phosphors and other luminescentmaterials for use in luminescent solar concentrator (LSC) or spectralconversion layer on thin-film solar cells devices that are known in theprior art.

It has been surprisingly found that certain inorganic crystalline hostmaterial comprising Tm²⁺ (e.g. Tm²⁺ doped phosphors) exhibit absorbingand luminescent properties that can be advantageously used in LSCdevices or SCL's. In particular, it has been found that the Tm²⁺ dopedinorganic crystalline materials may absorb the UV and visible part andat least part of the IR part of the solar spectrum (in total more than60% of the power from the sun) and have a sharp 4f-4f peak emission inthe infra-red around 1138 nm. As a result of the large Stokes' shift,the problem of self-absorption does not occur.

Moreover, the infrared emission peak at 1138 nm advantageously coincideswith the 1.13 eV bandgap for optimal conversion of the broad solarspectrum (in particular the unconcentrated AMI1.5 solar spectrum) on thebasis of a single-junction cell. As will be described hereunder in moredetail, this feature allows for improving the overall conversionefficiency of simple single-junction solar cells.

FIG. 1A-1C depict absorption, excitation and emission spectra of dopedinorganic materials according to various embodiments of the invention.In particular, FIG. 1A depicts the absorption spectra of Tm²⁺ dopedNaCl, NaI, CaI₂, NaBr and KBr. The spectra exhibit clear crystal-latticedependent d-band absorption in the VIS and IR. Furthermore, a weakabsorption peak of the parity forbidden 4f-4f absorption around 1138 nmis shown that is relatively independent of the type of host lattice.

In an embodiment, the luminescent material may comprise an inorganiccrystalline material (which may hereafter be referred to as theinorganic host material) that is doped with Tm²⁺. The inorganic hostmaterial may be a binary inorganic host material defined by the generalformula ML wherein M=Na,K,Rb,Cs and wherein L=Cl,Br,I. These materialsinclude (but are not limited to) NaCl, NaBr, Nal, KCl, KBr, KI, CsCl,CsBr, CsI, RbCl, RbBr, RbI, etc. These materials crystallize atrelatively low temperatures (e.g. room temperature) thereby allowingeasy formation of Tm²⁺ doped thin films.

The host materials may also be an alloy or mixture of the generalformula M1_((1-x))M2_((x))L with M1 and M2 any of the elements M, orML1_((1-x))L2_((x))with L1 and L2 any of the elements L. The hostmaterials may also be an alloy or mixture of the general formulaM1_((1-x))M2_((x))L1_((1-y))L2_((y)). The host materials may also be analloy or mixture of 3 or more elements M or L.

In another embodiment, a binary inorganic host material may be definedby the general formula NL₂ wherein N═Mg,Ca,Sr,Ba and L=Cl,Br,I,F. Thesematerials may include (but are not limited to) CaCl₂, CaBr₂, CaI₂,SrCl₂, SrBr₂, SrI₂, BaCl₂, BaBr₂, BaI₂, CaF₂, SrF₂, BaF₂.

The host materials may also be an alloy or mixture of the generalformula N1_((1-x))N2_((x))L₂ with N1 and N2 any of the elements N, orNL1_((2-2x))L2_((2x)) with L1 and L2 any of the elements L. The hostmaterials may also be an alloy or mixture of the general formulaN1_((1-x))N2_((x))L1_((2-2y))L2_((2y)). The host materials may also bean alloy or mixture of 3 or more elements N or L.

In an advantageous embodiment, binary iodine inorganic carrier materialsgiven by the general formula NI₂ N═Mg,Ca,Sr,Ba may be used as theluminescent inorganic converter material. The Tm²⁺ doped iodines exhibitsurprisingly good absorption characteristics for the solar spectrum.

In a particular advantageous embodiment CaI₂ or NaI may be used as Tm²⁺inorganic luminescent converter material. It has been surprisingly foundthat these materials exhibit superior absorption characteristics for thesolar spectrum.

In a further embodiment, a ternary inorganic host material may be used.These ternary materials may include (but are not limited to) CsCaCl₃,CsCaBr₃, CsCaI₃, RbCaCl₃, RbCaBr₃, RbCaI₃, CsSrBr₃, CsSrI₃, CsBaBr₃,CsBaI₃, RbSrI₃, KSrI₃.

It is submitted that the invention in not limited to the above-mentionedinorganic host materials. For example, in an embodiment the inventionmay also include mixed and/or alloyed forms of the above-mentionedinorganic host materials.

The spectra of FIG. 1A show clear 4f-5d (4f¹³→4f¹²(5d)¹) absorptionbands in the visible range of the spectrum, together with 4f¹³-4f¹³(²F_(7/2)→²F_(5/2)) absorption in the infrared. The latter absorptionbands are relatively independent of the type of inorganic host material.In contrast, the absorption by the 4f-5d transitions are stronglydepends on the type of inorganic host material in which Tm2+ is present.Mechanisms that determine the 4f-5d transition energy range aredetermined by the average (centroid) energy and the total crystal fieldsplitting of the 5d-configuration. It appears that the nearestcoordination shell of anions around Tm²⁺ determines both aspects.Crystal field splitting is determined by the size and shape of the anionpolyhedron. It scales with the the distance to the anions. The centroidshift is strongly determined by the chemical (covalence) and physical(polarizability) properties of the anion ligands.

The broadness of the absorption spectra of the different materials maybe further illustrated by the absorption edges of the materials. Therethe absorption edge may be determined as the wavelengths for with theabsorption is 25% of the maximum absorption of the lowest energy 5dstate of Tm²⁺. In the table hereunder the absorption edge for differentTm²⁺ doped inorganic materials together with their refractive indicesare provided:

Luminescent absorption refractive material edge [nm] index Tm²⁺:NaCl 7101.53 Tm²⁺:CaI₂ 844 1.78 Tm²⁺:NaI 831 1.74 Tm²⁺:NaBr 810 1.62 Tm²⁺:KBr763 1.54

From the table and the absorption spectra it follows that the Tm²⁺ dopedmaterials exhibit broadband absorption over a range between 200 nm and900 nm, preferably 220 and 880 nm, more preferably 240 and 860 nm.

FIG. 1B depicts the excitation spectra of the Tm²⁺ doped inorganicmaterials. The spectra were measured at an emission wavelength of 1138nm (i.e. at the top of the emission spectra as depicted in FIG. 1C).

FIG. 1C depicts the emission spectra corresponding to emissionassociated with the ²F_(5/2)→²F_(7/2) transition of Tm²⁺ doped NaCl,NaI, CaI₂, NaBr and KBr under 410, 412, 477, 480 and 409 nm excitationrespectively. As expected, the transition is relatively independent ofthe type of host lattice. Further, the graphs reveal that electrons thatare excited to a d-band state relax to the ²F_(5/2) state from whereradiative relaxation to the ²F_(7/2) state takes place. The resultingStokes-shift between absorption in the UV and visible range of thespectrum and the sharp peak emission in the infrared very well meets therequirements of a luminescent material that is suitable for LSC or SCLapplications.

The absorption spectra of these materials are substantially superiorwhen compared to absorption spectra of conventional luminescentmaterials such as dyes. FIG. 2 depicts the absorption spectra of theRed305 dye and the absorption spectrum of Tm²⁺ doped CaI₂. The solarspectrum as measured on the earth's surface is depicted in thebackground. The figure shows that the absorption edge of the dye isaround 605 nm whereas the absorption edge of Tm²⁺ doped CaI₂ is around844 nm. Further, the dye absorbs a 30% power fraction of the total solarintensity corresponding to a photon fraction (i.e. the percentage ofphotons absorbed) of only 18%, whereas Tm²⁺ doped CaI₂ has a powerfraction of around 63%, i.e. twice the amount of power compared to thedye, corresponding to a photon fraction of 44%. This figure thus showsthat the Tm²⁺ doped materials exhibit substantially superior LCSproperties when compared to the state-of-the-art dyes that are currentlyused for LCS.

FIG. 3 depicts the loss factor due to self-absorption for a Red305 dyeas a function of the LSC radius for different LQEs (the loss factorequals 1 minus the transmitted fraction). The fraction of the initiallyemitted energy that is transmitted through the edges of a circular LSCwith a refractive index of 1.5 is depicted for different radii and LQEs.This figures shows that even for a dye with an LQE of 100%, only 20% ofthe luminescent light is transmitted out of the edges of an LCS with aradius of around 0.56 meter. When lowering the LQE to 80%, only 10% ofthe luminescent light is transmitted out of the edges of the LCS. Thistrend is also valid for other dyes or luminescent materials that arebased on nano-particles and that exhibit self-absorption. These lossesdue to self-absorption are eliminated by using luminescent Tm²⁺ dopedinorganic materials according to the invention in an LSC structure.

The syntheses of Tm²⁺ doped materials is problematic as often Tm³⁺instead of Tm²⁺ sites are formed in the inorganic host material. Forexample, the use of a conventional firing process in a reducing N₂/H₂atmosphere does not result in stable Tm²⁺ doped materials. Instead, theTm³⁺ state is formed. For example, heating a mixture of the salt andTmI₂ or TmCl₃ in an alumina crucible in a tube furnace with N₂/H₂atmosphere at different temperatures (below and above the melting pointof the salts) resulted in the formation of Tm₂O₃ or Tm³⁺ doped CaI₂ orNaCl (according to XRD analyses) and the diffuse reflectance spectrashowed the presence of Tm³⁺ rather than Tm²⁺.

Therefore, in order to obtain Tm²⁺ doped inorganic materials thatexhibit the above-described advantageous absorption-emissioncharacteristics, a mixture of an inorganic host material (e.g. NaCl) anda Tm²⁺-based salt (e.g. TmI₂) was contained in a closed quartz ampoulethat was under vacuum. The doping concentration of the Tm²⁺ may bevaried on the basis of the amount of Tm²⁺-based salt in the mixture.Heating the ampoule in a furnace however resulted in the formation ofTm³⁺ formation and no Tm²⁺ was found. However, heating the ampoule (withthe mixture inside) with a gas burner (a burner that is normally used tomelt the quartz) resulted in the formation of Tm²⁺ doped NaCl. On thebasis of this process, different Tm²⁺ doped inorganic materials weresuccessfully synthesized, including (but not limiting to) Tm²⁺ dopedNaBr, NaI, KBr, CaCl₂ and CaI₂. Heating a closed vacuum-pumped ampoulehad a negative effect on the amount of Tm²⁺ in the final product.Therefore, during the melting process the ampoule was connected to avacuum pump, which ensured that the pressure during the firing of thematerials was around 10⁻⁴ mbar or lower. The samples made in this waywere all black or greenish black and showed the desired Tm²⁺ f-femission. Besides the Tm²⁺ doping, only very small amounts of Tm³⁺ waspresent in some samples. On the basis of this recipe various Tm²⁺ dopedinorganic materials were fabricated. For example, for the synthesis ofcirca 1.5 g of 3% Tm doped CaI₂ the following recipe may be used:

-   -   mixing 1.425 g of CaI₂ with 0.063 g of TmI₂ inside a N₂ filled        glove-box;    -   transferring the mixture into a dried quartz ampoule;    -   closing the ampoule inside the N₂ filled glove-box with a valve;    -   connecting the ampoule, with mixture and N₂ gas inside, to a        vacuum pump;    -   vacuum pumping the ampoule;    -   heating the ampoule with a burner during vacuum pumping until        the mixture melts (typically within 1 to 2 minutes).    -   stopping the heating, closing the ampoule by melting it, and        disconnecting the ampoule from the pump once the quartz has        cooled down.    -   opening the ampoule in the glove-box by breaking the quartz;        and,    -   forming a powder of the crystalline material.

On the basis of this fabrication methods Tm²⁺ doped luminescentmaterials may be fabrications wherein the doping concentration of saidTm²⁺ ions in said host material may vary selected between 0.1 and 100%depending on the ratio between the amount of Tm²⁺-based salt and theinorganic host material. In an embodiment, the ratio may be selectedsuch that the concentration of said Tm²⁺ ions is between 1 and 50%, morepreferably 1 and 30%. Hence, from the above, it follows that stable Tm²⁺doped crystalline materials in powder form may be synthesized by meltinga stoichiometric mixture of salts under vacuum conditions, preferably ata pressure of 10⁻⁴ mbar or less. The crystalline material in powder formmay be used in the formation of LSC devices, which will be describedhereunder in more detail with reference to FIG. 9A-9C.

In some situations, it may be advantageous to use a material synthesisprocess that is compatible with convention thin-film semiconductorprocessing technologies so that the formation of crystalline Tm²⁺ dopedinorganic materials may be used together with other processing and/ormaterial deposition steps. Hence, in addition to the above-describedfiring process for producing powder-based Tm²⁺ doped inorganicmaterials, crystalline Tm²⁺ doped inorganic materials were alsosynthesized on the basis of a semiconductor deposition technique. Inparticular, crystalline thin-film Tm²⁺ doped layers were realized usinga sputtering technique. In an embodiment, an RF magnetron co-sputteringtechnique may be used. In the co-sputtering technique, at least twotargets may be used, e.g. a first target comprising an inorganic hostmaterial and a second Tm target.

Successful polycrystalline Tm²⁺ doped thin-films of a thickness between1 and 5 micron were grown on a suitable carrier substrate such as SiO₂(quartz), Al₂O₃ or various types of glass under different sputteringconditions. In this particular example NaCl was used as the inorganichost material but other binary, ternary or quaternary host materials asdescribed with reference to firing process may also be used withoutdeparting from the invention. The pressure during the sputtering processmay be selected between 1 and 5 mTorr. The NaCl target was set at an RFpower selected between 20 and 50 W (corresponding to a rate between 1and 5 Å/s) and the Tm target was set at a DC power between 10 and 40 W(corresponding to a rate between 0.05 and 0.08 Å/s). On the basis ofthese setting, NaCl doped Tm²⁺ films wherein the Tm²⁺ dopant percentageis between 0.2 and 12% were realized (these values were determined onthe basis of EDX measurements). Here a dopant percentage of 1% meansthat 1% of the cations in the anorganic host material is replaced by aTm²⁺ cation.

FIGS. 4A and 4B show SEM pictures of crystalline Tm²⁺ doped thin-filmlayers according to various embodiments of the invention. In particular,FIGS. 4A and 4B show SEM pictures of crystalline Tm²⁺ doped NaClthin-film of a thickness of around 2 micron that were deposited onto anSiO₂ substrate at a temperature of 200° C. and 300° C. respectively. Thepresence of Tm²⁺ in the NaCl film gives a greyish/tinted appearance(NaCl films are transparent). A similar effect was observed with thepowders that turn from white to greyish/black when doped with Tm²⁺.Polycrystalline films were grown wherein the grain size may becontrolled by temperature. As shown in FIG. 4A, when using a depositiontemperature 200° C. an average grain size between 200 and 400 nm may berealized. At a deposition temperature of 300° C. the average grain sizeis between 600 and 1000 nm as shown in FIG. 4B.

FIG. 5 depicts an XRD measurement of a sputtered Tm²⁺ doped NaCl thinfilm according to an embodiment of the invention. The locations of thepeaks in the spectrum match those of NaCl. FIG. 6 depict excitationspectra of an Tm²⁺:NaCl thin-film and of a Tm²⁺:NaCl powder sample. Thespectra are recorded while monitoring the Tm²⁺ line emission around 1140nm. This figure shows that both the powder and thin-film materialscomprise Tm²⁺ that is responsible for absorption of light of the solarspectrum by one or more of its 5d energy bands. Further, it shows thatif a Tm²⁺ ion is excited in one of its 5d energy bands by photons of thesolar spectrum, a radiation-less relaxation to a lower-level f bandtakes places before a line emission of photons at around 1138 nm takesplace. Hence, the Tm²⁺ doped thin-films exhibit a large Stokes-shift sothat no overlap between emission and absorption is present.

FIG. 7 depicts the emission spectrum of Tm²⁺:NaCl thin-film and of aTm²⁺:NaCl powder sample according to an embodiment of the invention. Thespectra were recorded under excitation at 415 nm. This graph shows thatfor both the powder and thin-film samples, the Tm²⁺ sites areresponsible for the photon emission.

FIG. 8 depicts a fit of transmission spectrum of a Tm²⁺ doped NaCl filmaround the Tm²⁺ absorption region. To quantify the absorption strength,the transmission excluding the dip is fitted and the amplitude of thedip with respect to this fitted line is determined. The inset shows theresulting relative absorption.

Alternative sputtering techniques may be used without departing from theinvention. For example, in an embodiment a single target comprising aTm²⁺ doped target material that was synthesized using the firing methodas described above. For example, an Tm²⁺:NaCl or Tm²⁺:CaI₂ powder may besynthesized and pressed into a tablet that can be inserted into thetarget of a sputtering system.

FIG. 9A-9C depict schematic cross-sections of solar radiation conversiondevices, in particular luminescent solar energy concentrator devices,comprising a Tm²⁺ doped inorganic material according various embodimentsof the invention. FIG. 9A depicts a LSC device according to anembodiment of the invention. The LSC comprises a waveguide structure 902comprising a first (top) surface 903, a second (bottom) surface 905 andone or more edges 906. At least part of the edges 906 of the waveguidestructure may be coupled to a photovoltaic cell 908. In this embodiment,the waveguide structure may be formed of a transparent matrix materialin which particles 910, preferably nano-scale particles, of a Tm²⁺ dopedinorganic crystalline material as described in this specification areembedded. When photons 912 from the solar spectrum that enter thewaveguide structure are absorbed by one or more 5d energy bands of theTm²⁺ sites, the excited Tm²⁺ sites transmit photons 914 via an ff-lineemission at a wavelength of around 1138 nm. The emitted photons may beguided via the waveguide structure to the edges of the waveguide where aphotovoltaic cell 908 that is optimized for converting the photons thatare emitted by the Tm2+ ions into electrical power.

In an embodiment, the photovoltaic cell may be a Copper Indium Gallium(di)Selenide (CIGS) photovoltaic cell. In another embodiment, thephotovoltaic cell may be an Copper Indium (di)Selenide (CIS)photovoltaic cell. These materials are very efficient for converting theemitted 1138 nm (near)infrared solar radiation of the Tm²⁺ sites intoelectrical energy.

In a further embodiment, the photovoltaic cell may comprise a NIR/IRabsorbing organic active layer or a NIR/IR dye-sensitized active layer.In an embodiment, the photovoltaic cell may comprise an organicsemiconducting layer, e.g. MEH-PVV, that is sensitized with NIR/IRabsorbing quantum dots. For example, by controlling the size of low-bandgap (binary) semiconductors (e.g. PbS, PbSe, InAs and/or HgTe) quantumdots, the quantum dots may be tailored to absorb in the (near) infraredspectrum between 900 and 2000 nm. See e.g. Sargent et al in“Solution-based Infra-Red Photovoltaic Devices, Nature Photonics 3,325-331 (2009). In another embodiment, the photovoltaic cell maycomprise a (single) walled carbon nano-tube layer or a graphene activeNIR/IR absorbing layer.

The Tm²⁺ doped particles may be embedded in a transparent matrixmaterial comprising a transparent organic polymer that has excellenttransmittance properties in the near-infrared range of the opticalspectrum such as poly(methyl methacrylate) (PMMA) or a polycarbonate. Inan embodiment, the refractive index of the matrix material may beselected to substantially match the refractive index of the Tm²⁺ dopedparticles so that losses due to scattering of the emitted light out ofthe waveguide structure is minimized.

FIG. 9B depicts a LSC device according to another embodiment of theinvention. The LSC comprises a waveguide structure 902 comprising afirst (top) surface 903, a second (bottom) surface 905 and one or moreedges 906. At least part of the edges 906 of the waveguide structure maybe coupled to a photovoltaic cell 908. The waveguide structure may beformed of a transparent high-index organic polymer or glass. At leastpart of the top and/or bottom surface of the waveguide structure may becovered with a luminescent layer 916 that may comprise a Tm²⁺ dopedinorganic crystalline material as described in this specification.

In an embodiment, the thin-film luminescent layer may be formed of atransparent matrix material in which particles, preferably nano-scaleparticles, of a Tm²⁺ doped inorganic crystalline material are embedded.Alternatively, the optically active layer may be Tm²⁺ doped(poly)crystalline thin-film layer that is formed on the waveguidestructure. The thin-film layer may be formed onto the waveguidestructure using e.g. a (co-)sputtering method as described above. Theuse of a (poly)crystalline layer that is coupled as an optically activelayer to the waveguide structure provides the advantages that the effectof scattering that may occur when using a matrix layer comprising Tm²⁺doped particles is eliminated.

In a further embodiment (not shown), both at least part of the topsurface and the bottom surface of the waveguide structure may be coveredwith a thin-film luminescent layer. This way, sunlight that passes thetop and bottom side of the waveguide structure may be converted into(near) infrared light that is guided by the waveguide structure towardsthe photovoltaic cell.

FIG. 9C depicts a LSC device according to yet another embodiment of theinvention. In this embodiment, the LSC device comprises one or moreluminescent layers that are similar to the ones described with referenceto FIG. 9B. In this embodiment however, the luminescent layer 916 isembedded in the waveguide structure 902.

It is submitted that the devices depicted in FIG. 9A-9C are non-limitingexamples embodying the invention and many variations and/or combinationsof features of these embodiments are possible without departing theinvention. For example, in an embodiment, a solar radiation conversiondevice may comprise a waveguide structure comprising Tm²⁺ dopedluminescent layers that are provided over the first and/or secondsurface of the waveguide structure and embedded in the waveguidestructure. In a further embodiment, at least part of the photovoltaiccell may be part of the waveguide structure.

FIGS. 10A and 10B depict cross-sections of schematic solar radiationconversion devices according to another embodiment of the invention. Inparticular, FIG. 10A depicts a solar radiation conversion devicecomprising a broadband-absorbing Tm²⁺ luminescent conversion layer 1002and a photovoltaic device 1004 comprising a first surface 1001 andsecond surface 1003. Depending on the configuration, the first (top)surface may be a light receiving surface. In case of a transparent solarcell, both the first and second surface may be light receiving surfaces.The conversion layer may be coated or deposited on the first and/orsecond surface. In an embodiment, the conversion layer may be athin-film (poly)crystalline Tm²⁺ doped luminescent material as describedin this disclosure with reference to FIG. 4-8. In another embodiment,the conversion layer may be a transparent matrix material comprisingparticles, preferably nano-particles, of a Tm²⁺ doped luminescentmaterial as descried in this disclose.

In an embodiment, the photovoltaic device may be silicon photovoltaicdevice, e.g. an micro-crystalline (thin-film) silicon photovoltaicdevice. In an another embodiment, the photovoltaic device may be anNIR/IR photovoltaic cell (single or multi-junction) for converting atleast the (near)infrared part of the solar spectrum 1006 into electricpower. Further, light from the UV and visible part 1008 of the solarspectrum is converted by the broadband-absorbing Tm²⁺ luminescentconversion layer 1002 into 1138 nm infrared solar radiation.

FIG. 10B depicts a solar radiation conversion device, e.g. a solar sell,comprising a broadband-absorbing Tm²⁺ luminescent material. The devicemay comprise a substrate 1012, preferably a transparent substrate,comprising a multi-layered thin-film photovoltaic device. Thephotovoltaic device may comprise a first contact layer 1014 formed on afirst surface of the substrate. A solar radiation absorbing active layeror multilayer 1018 is formed over the first contact layer and a secondlayer 1016 is formed over the active layer.

In an embodiment, the photovoltaic device may comprise at least onelayer comprising infrared absorbing quantum dots (i.e. nano-particleshaving a size such that quantum-confinement effects are induced in theparticles, i.e. reducing the size of the particles to less than the Bohrradius of the electron and hole functions in the semiconductor) andnano-particles of a broadband-absorbing Tm²⁺ luminescent material asdescribed in this disclosure. In an embodiment, the size of at leastpart of the infrared absorbing quantum dots may be optimized forabsorption of infrared solar radiation that is transmitted by the Tm²⁺luminescent nano-particles. For example, PbS quantum dots that have aparticle size of around 3-4 nm will absorb (near) infrared radiationaround 900-111 nm. Hence, in this embodiment an infrared part 1006 ofthe solar radiation may be absorbed directly by infrared absorbingquantum dots while the UV and the visible part of the solar radiationmay be converted by the Tm²⁺ luminescent nano-particles into infraredsolar radiation 1010 of a wavelength of around 1138 nm. This way, theoverall conversion efficiency of a simple infrared photovoltaic devicemay be extended on the basis of visible solar radiation that isconverted into infrared solar radiation.

In another embodiment, the photovoltaic device may comprise furthercomprise a further broadband-absorbing Tm²⁺ luminescent(poly)crystalline thin-film layer. Such layer may be positioned betweenthe substrate and the first contact layer in order to increase thetransformation of the radiation of the UV/visible part of the solarspectrum into radiation of the infrared part of the solar spectrum.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

The invention claimed is:
 1. A solar radiation concentrating device comprising: a transparent waveguide structure comprising a top surface, a bottom surface and one or more edges, the top surface being configured to receive solar radiation; and a photovoltaic device coupled to at least one of the one or more edges of the transparent waveguide structure; the transparent waveguide structure comprising a luminescent thin-film layer provided over a transparent substrate, the luminescent thin-film layer including a luminescent Tm²⁺ doped inorganic material exhibiting broadband absorption of light in UV range of solar radiation and broadband absorption of light in visible range of the solar radiation, wherein at least 60% of the solar radiation is absorbed by the luminescent layer; wherein the solar radiation absorbed by the luminescent layer is emitted by the luminescent layer as infrared radiation having a wavelength of between about 1100 nm and about 1200 nm; and wherein the transparent waveguide structure is configured to guide the infrared radiation to the photovoltaic device which is configured to convert at least part of the infrared radiation into electrical power; and wherein host material of the luminescent Tm²⁺ doped inorganic material consists of binary or quaternary inorganic crystalline host material.
 2. The solar concentrating device according to claim 1, wherein the host material is binary inorganic crystalline host material.
 3. The solar concentrating device according to claim 2, wherein the Tm²⁺ ions are present in the host material in a concentration between 0.1 and 100%.
 4. The solar radiation conversion device according to claim 2, wherein the binary inorganic crystalline host material is defined by the general formula ML, wherein M=Na,K,Rb,Cs and L=CI,Br,I,F; or, wherein the binary inorganic crystalline host material is defined by the general formula NL₂ wherein N=Mg,Ca,Sr,Ba and L=CI,Br,I,F; or.
 5. The solar radiation conversion device according to claim 1, wherein the luminescent thin-film layer is a (poly)crystalline thin-film layer.
 6. The solar concentrating device according to claim 1, wherein the luminescent layer is embedded in the transparent waveguide structure.
 7. The solar concentrating device according to claim 1, wherein at least part of the luminescent layer is provided over a light-receiving face of said photovoltaic device.
 8. The solar concentrating device according to claim 1, wherein photovoltaic device comprises the Tm²⁺ based inorganic material.
 9. The solar concentrating device according to claim 1, wherein the photovoltaic device comprises an infrared absorbing active layer, the infrared absorbing active layer comprising at least one of: a type IV, III-V, or II-VI semiconductor compound, copper indium gallium (di)selenide (CIGS), copper indium (di)selenide (CIS), infrared absorbing quantum dots, an infrared absorbing polymer, graphene or (carbon) nanotubes.
 10. The solar concentrating device according to claim 1, wherein the Tm²⁺ ions are present in a concentration between 1% and 50%.
 11. The solar concentrating device according to claim 1, wherein the Tm²⁺ ions are present in a concentration between 1% and 30%.
 12. The solar concentrating device according to claim 1, wherein the Tm²⁺ ions are present in a concentration between 0.2% and 11%.
 13. The solar concentrating device according to claim 1, wherein the luminescent layer is a sputtered or a co-sputtered Tm²⁺ doped thin-film layer.
 14. The solar concentrating device according to claim 1, wherein the luminescent layer comprises a matrix material in which Tm²⁺ doped particles are embedded.
 15. The solar concentrating device according to claim 14, wherein the Tm²⁺ doped particles have average dimensions between 1 and 1000 nm.
 16. The solar concentrating device according to claim 14, wherein the matrix material is a transparent organic polymer.
 17. The solar concentrating device according to claim 16, wherein the transparent organic polymer is a poly(methyl methacrylate) (PMMA) or a polycarbonate.
 18. A solar radiation concentrating device comprising: a transparent waveguide structure comprising a top surface, a bottom surface and one or more edges, the top surface being configured to receive solar radiation; and a photovoltaic device coupled to at least one of the one or more edges of the transparent waveguide structure; the transparent waveguide structure comprising a luminescent layer, the luminescent layer including particles of a luminescent Tm²⁺ doped inorganic material having average dimensions between 1 and 1000 nm, the luminescent Tm²⁺ doped inorganic material exhibiting broadband absorption of light in UV range of solar radiation and broadband absorption of light in visible range of the solar radiation; wherein solar radiation absorbed by the luminescent layer is emitted by the luminescent layer into infrared radiation having a wavelength of between about 1100 nm and about 1200 nm; wherein the transparent waveguide structure is configured to guide the infrared radiation to the photovoltaic device which is configured to convert at least part of the infrared radiation into electrical power; and wherein host material of the luminescent Tm²⁺ doped inorganic material consists of binary or quaternary inorganic crystalline host material.
 19. The solar concentrating device according to claim 18, wherein the infrared radiation has a peak emission at a wavelength of around 1138 nm.
 20. The solar concentrating device according to claim 1, wherein the infrared radiation has a peak emission at a wavelength of around 1138 nm.
 21. A solar radiation concentrating device comprising: a transparent waveguide structure comprising a top surface, a bottom surface and one or more edges, the top surface being configured to receive solar radiation; and a photovoltaic device coupled to at least one of the one or more edges of the transparent waveguide structure; the transparent waveguide structure comprising a luminescent layer, the luminescent layer including a luminescent Tm²⁺ doped inorganic material exhibiting broadband absorption of light in UV range of solar radiation and broadband absorption of light in visible range of the solar radiation; wherein the UV light and visible light absorbed by the luminescent layer is emitted by the luminescent layer as infrared radiation having a wavelength of between about 1100 nm and about 1200 nm; wherein the transparent waveguide structure is configured to guide the infrared radiation to the photovoltaic device which is configured to convert at least part of the infrared radiation into electrical power; and wherein host material of the luminescent Tm²⁺ doped inorganic material consists of binary or quaternary inorganic crystalline host material.
 22. The solar concentrating device according to claim 21, wherein the infrared radiation has a peak emission at a wavelength of around 1138 nm. 